(a) Field of the invention
The present invention relates to an audio amplifier.
In audio frequency equipment it is desirable to be able to amplify a low level audio signal by means of an amplifier to a large level without introducing any unwanted noise. The input stage of the amplifier may be provided by a number of arrangements. Any such arrangement, however, should enable the non-inverting input to be biased at ground level. Having the non-inverting input biased at ground level obviates the need for any d.c. blocking capacitor and furthermore enables a controlling component such as a variable impedance to be used which is normally referenced to ground potential. An arrangement at the input stage should also enable the inverting input to be biased not at ground potential since the inverting input is always connected to a feedback network in order to define the closed loop gain as well as the output d.c. level.
Description of the Prior Art
Known devices have employed a cascade differential pair at the input stage since it meets the aforesaid requirements and also provides a relatively low noise.
A cascade differential pair consists, for example, of a pair of transistors (or compound transistors) of opposite polarities whose emitters are connected together.
Alternatively a common emitter differential ("long-tailed") pair has been used with a level shift transistor at its non-inverting input. However, the unity gain of the level shift transistor greatly degrades the noise performance of the amplifier.
Advantageous parameters required by the input stage of an audio amplifier are as follows: (1) the non-inverting input must be ground compatible; (2) the inverting input must be non-ground compatible; (3) relatively low noise; (4) a high output impedance; (5) a high ripple rejection; (6) and an inverting input/output characteristic.
Previously proposed audio amplifiers incorporating a cascade differential pair at the input stage have not met all of the above criteria. For example FIG. 1 of the accompanying drawings illustrates an audio amplifier using a cascade differential pair. Transistor Q9 and the cascade differential pair (Q1 and Q2) form the input stage of the amplifier. Current source CC100 and current mirror transistors Q106 and Q9 are major noise contributors. The input stage has a high output impedance but does not have an inverting input/output characteristic. Hence the amplifier requires two more inverting gain stages (Q100 and Q101), which increases the noise contribution and means that an external capacitor is required to stabilize the amplifier.
Another known audio amplifier is shown in FIG. 2 of the accompanying drawings. The input stage is formed by a cascade differential pair (Q1 and Q2) and a resistive load R100. The noise from the input stage is low but it provides low output impedance and has a non-inverting input/output characteristic. Accordingly a non-inverting gain stage is required and is provided by transistors (Q102, Q103, Q104 and Q105) forming a two gain stage amplifier. Again an external capacitor is required because the Miller effect compensation is not sufficient for the non-inverting gain stage.
Another known audio amplifier is shown in FIG. 3 of the accompanying drawings, where the input stage of the amplifier comprises a cascade differential pair Q1, Q2; a current mirror formed by transistors Q3, Q4 and resistors R1, R2; and an active load in the form of a constant current source provided by a transistor Q5, resistors R3 to R6, and diode-connected transistors Q6, Q7. This input stage provides a high output impedance, low noise and an inverting input/output characteristic. The input stage enables the use of a single further gain stage formed by a common emitter transistor Q8 with a constant current source CC1 as the collector load. The amplifier thus has only two gain stages and a single integrated compensating capacitor C1 is sufficient to ensure stability, thus obviating the need for any external compensating capacitor to stabilize the amplifier. However the amplifier does not meet all of the aforesaid requirements. The resistor R4 is provided instead of a constant current source previously used. Although resistor R4 reduces the noise contribution, it injects ripple into the amplifier. In some cases the power supply rejection ratio can be as low as 0 dB.